1. Field of the Invention
This invention relates generally to methods and apparatuses for preventing negative voltages, and in particular embodiments, to a method and apparatus for eliminating motor negative voltages during motor braking for the purpose of sensing motor commutation pulses.
2. Description of Related Art
Commutation pulses are generated, for example, in direct current permanent magnet motors. As the motor rotates, the current through its windings is periodically interrupted due to the action of the motor brushes. This produces steep pulses or spikes in the motor current, called commutation pulses. By detecting and counting motor commutation pulses, it is possible to track the position of a motor.
Typically motors are driven and braked with H-bridge circuits. Negative voltages arise during motor braking, when braking is accomplished using a standard H-bridge motor drive circuit. While a motor is braking, sensing of motor commutation pulses can be difficult, because negative voltages substantially cannot be tolerated with some junction isolated technologies. For instance, use of some junction isolated technologies as part of a commutation pulse sensing circuit results in the sensing circuit intermittently detecting commutation pulses during braking.
A standard H-bridge circuit is illustrated in FIGS. 1A and 1B. In FIG. 1A, a first 10, a second 12, a third 14, and a fourth 16 bipolar junction transistors are shown coupled to a motor 18. A collector 20 of the first transistor 10 and a collector 22 of the second transistor 12 are each coupled to a high potential terminal 24, at which, for example a positive voltage is applied. An emitter 26 of the third transistor 14 and an emitter 28 of the fourth transistor 16 are coupled to a low potential terminal 30, e.g. ground. A first terminal 32 of the motor 18 is coupled to an emitter 34 of the first transistor 10 and to a collector 36 of the third transistor 14. A second terminal 38 of the motor 18 is coupled to an emitter 40 of the second transistor 12 and to a collector 42 of the fourth transistor 16. A first parasitic diode 44 is coupled effectively between the collector 36 and the emitter 26 of the third transistor 14. Similarly, a second parasitic diode 46 is coupled effectively between the collector 42 and the emitter 28 of the fourth transistor 16.
The first transistor 10 has a base 48, the second transistor 12 has a base 50, the third transistor 14 has a base 52, and the fourth transistor 16 has a base 54. These bases are controlled by control signals (not shown).
To operate the H-bridge circuit of FIG. 1A in a drive mode (in which the motor behaves as a load), the base 48, that is the base-emitter junction 48, 34, of the first transistor 10 and the base 54, that is the base-emitter junction 54, 28, of the fourth transistor 16 are forward biased, such that current can flow from their respective collectors to their respective emitters. The second transistor 12 and the third transistor 14 have their bases 50 and 52, respectively, biased such that these transistors do not conduct current. In this drive mode, as illustrated in FIG. 1A, a motor current 55 flows from the high potential terminal 24 through the first transistor 10, through the motor 18, and through the fourth transistor 16 to the low potential terminal 30.
In order to brake the motor 18, typically a low side brake mode is used, as shown in FIG. 1B. As part of that brake mode, the base 48 of the first transistor 10 is no longer forward biased, thus decoupling the first terminal 32 of the motor 18 from the high potential terminal 24. In addition, the base 52 of the third transistor 14 is forward biased to couple the first terminal 32 to the low potential terminal 30, through the third transistor 14. The control signals applied at the base 50 of the second transistor 12 and applied at the base 54 of the fourth transistor 16 remain unchanged relative to the drive mode.
As a result of these control signals, the motor 18 acts as a generator and the motor current 55 reverses direction and flows in the direction of the arrow illustrated in FIG. 1B. Due to the direction of the current 55 flow (counterclockwise in FIG. 1B), a negative voltage is generated at a terminal 56. That negative voltage at terminal 56 is caused by the voltage drop across the second parasitic diode 46.
Terminal 56 is a convenient place at which to sense motor commutation pulses. However, as mentioned above, for some junction isolated technologies which are used in commutation pulse sensing circuits, there exists the problem of substantial intolerance to negative voltages. Thus, there is a need for eliminating negative voltages, such that a designer is not prevented from using some junction isolated technologies.